High-speed digital copiers and printers require temporary storage for images prior to printing them. This image storage subsystem serves two important purposes. First, it decouples the speed at which input images are acquired (scanned or rasterized) from the speed at which they are printed. Second, the temporary storage allows multiple copies of a document to be produced without having to re-acquire the input images; i.e., rescan the document or in the case of an input from a computer rerasterize the data from a coded form or object form used in a page description language. For high-volume printing, where multiple sets of large documents need to be produced, the temporary storage is most economically implemented using disk drives. Compressing the images before they are stored on the disks can further increase the capacity of the temporary storage.
A system using disk drives to store images prior to printing them is disclosed in U.S. Pat. No. 5,142,667 to Dimperio et al. The patent describes a system which uses several disk drives to implement a disk memory which is used to store images prior to subsequent processing or printing. Dimperio et al. describe various experiments and algorithms for determining the throughput of the system based on the disk bandwidth, but do not determine the minimum disk bandwidth required for full output productivity.
U.S. Pat. No. 5,495,339 to Stegbauer et al. similarly discloses a disk memory for image storage. This patent also discloses the use of a resource manager to schedule the use of the disk drives. Since the disclosed system may not have sufficient disk bandwidth to read images in time to optimally print them, the resource manager determines when it is necessary to reduce output productivity by inserting a print pitch skip. A similar resource management approach is disclosed by May et al. in U.S. Pat. No. 5,375,202. Both of these resource managers require predicting when a series of disk access operations will be completed in the future. Such predictions are difficult to make given the variability of disk access times and the inability to predict when disk soft errors will occur.
A system that uses image compression in conjunction with a disk memory is disclosed in U.S. Pat. No. 5,130,809 to Takayanagi. By using a compression algorithm that operates at a constant compression rate, the size of the image is reduced and the disk bandwidth required to store the image is similarly reduced. However, to achieve the constant compression rate disclosed by Takayanagi, a non-lossless compression algorithm, such as block approximation or adaptive prediction coding, must be used. This results in reduced image quality when the image is subsequently printed, since the decompressed image is not identical to the original input image.
U.S. Pat. No. 5,611,024 to Campbell et al. discloses the use of a lossless image compression algorithm to reduce the amount of memory required to store an image. The compressed images are stored in solid-state memory that is intended to store only a small number of pages. The system disclosed by Campbell et al. does not use disk drives to achieve the capacity required for high-volume printing in which large numbers of images must be stored to maximize output productivity. Without using disk drives, the images would have to be stored in solid-state memory, which is considerably more expensive.